xhci: Add missing parameter description to xhci_get_endpoint_index()

xhci: Add missing parameter description to xhci_get_endpoint_index()

Fix kernel-doc warning by documenting the @desc parameter:

drivers/usb/host/xhci.c:1369: warning: Function parameter or struct member 'desc' not described in 'xhci_get_endpoint_index'

Add detailed description of the @desc parameter and clarify the indexing
logic for control endpoints vs other types. This brings the documentation
in line with kernel-doc requirements while maintaining technical accuracy.

Signed-off-by: Hans Zhang <18255117159@163.com>
Link: https://lore.kernel.org/r/20250506033101.206180-1-18255117159@163.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>